Recently, composite material has been recognized as one of the high performance materials, and as a reinforcement for the composite material, there have been developed graphite fibers and wholly aromatic polyamide fibers such as poly(p-phenyleneterephthalamide) fibers, which have high elasticity and tenacity. These reinforcing fibers are, however, long fibers so that they reinforce the composite material only in one direction. To reinforce the composite material in at least two directions, at least two sheets of the composite material having different reinforcing directions should be laminated. In addition, it is difficult to isotropically reinforce the composite material with conventional reinforcement fibers. Consideration can be given to cutting the long fiber to produce short fibers. However, it is impossible to produce a short fiber having a length of about 100 .mu.m or less. These drawbacks can be overcome by use of a polymeric whisker, the fiber length of which can be adjusted by controlling the deposition rate of the crystal. Hitherto, as an organic polymeric whisker, only the poly(oxymethylene) whisker is known. The poly(oxymethylene) whisker is prepared by a method comprising ionic ring opening polymerization of trioxane in hexane and simultaneous precipitation of a produced polymer. Although the whisker having a length of 30 to 150 .mu.m and a diameter of 1 to 3 .mu.m is obtained, its melting point is as low as 183 to 185.degree. C. Therefore, it finds very few applications, for example as, a reinforcement of material used as a speaker cone.
As one of organic polymers having high melting points, poly(p-oxybenzoyl) is known and prepared by heating p-acetoxybenzoic acid in a heating medium (cf. J.P.S., Polymer Letters Ed., 22, 433 (1984)). The polymeric crystal produced by this process is, however, of a cylindrical or flat-plate shape, which does not have a sufficiently large ratio of length and diameter to be used as reinforcement for composite material.
It has been reported that a fibril form polymer was produced by polymerizing p-acetoxybenzoic acid in a heating medium under specific conditions (cf. Polymer Preprints, Japan, 32(4), 848). The obtained fibril form polymer has not been used as the reinforcement of the composite material since fibrils tend to intertangle with each other and cannot be thoroughly dispersed in a matrix polymer to be reinforced.
Production of poly(2-oxy-6-naphthoyl) has not been reported.